As computer-related equipment has had its performance further enhanced and its sizes further reduced and as information network equipment has become even more popular these days, it becomes increasingly necessary to realize even higher-density mounting with a printed circuit board for use in those types of equipment. In the past, a quad flatpack package (QFP) with lead terminals around its component would often be used as a member to realize high-density surface mounting. Recently, however, a ball grid array (BGA), which is relatively small in size and which can cope with multiple-pin applications, is used more and more often. The BGA may also be used as a spacer member when a quartz oscillator and a temperature-compensating IC are stacked one upon the other.
As shown in FIGS. 2(a) and 2(b), a ball grid array (BGA) is an LSI package in which solder balls 50 are bonded onto the lower surface of an LSI chip with an interposer 62 interposed between them. The solder balls 50 are arranged in matrix on one surface of the interposer 62, and are used as input/output terminals for the package. Each of these solder balls 50 is a tiny sphere with a diameter of about 0.1 mm to about 1.0 mm, and may be obtained by forming a solder layer on the surface of a metallic ball, for example.
If the solder layer of a lead-tin based material is deposited by an electroplating technique, then voids may be created in the solder layer while the solder balls are being heated, melted and bonded onto the pads of the interposer, which is a serious problem. The reason is that once those voids have been created, the interposer and the solder balls are either connected defectively or misaligned from each other, thus affecting the reliability of the BGA.
The applicant of the present application discovered that those voids were created by the hydrogen gas that was absorbed in the solder layer being formed by the electroplating technique, and could be minimized by reducing the absorption of that hydrogen gas. Based on this discovery, the applicant of the present application disclosed a method for minimizing the creation of voids by reducing the quantity of hydrogen gas absorbed into the solder layer with the ion concentrations of lead and tin in the plating solution and the current density during the electroplating process controlled (see Japanese Patent Application Laid-Open Publication No. 10-270836, pages 2 and 3, in particular).
In recent years, solder with lead is being replaced with solder with no lead (which is also called “Pb-free solder”). As the Pb-free solder, an Sn—Ag based solder or an Sn—Ag—Cu based solder is used, for example.